This invention relates to a method for regenerating the activity of an alumina catalyst which has been poisoned through use in a modified Claus plant reactor by condensed sulfur, carbon deposits, aluminum sulfate and, possibly, aluminum nitrate, and which has suffered a loss in surface area.
The effective removal of sulfur compounds from sour natural gas is of prime importance in the gas industry. Natural gas, consisting mainly of methane with traces of ethane, propane and higher hydrocarbons, is oftentimes contaminated with hydrogen sulfide gas. The hydrogen sulfide is commonly removed from the natural gas by selective absorption into an amine solution such as ethanolamine, and is subsequently stripped from the solution by a flashing procedure. The gas thus evolved is rich in hydrogen sulfide but still contains some hydrocarbons and carbon dioxide, as well as traces of amine.
The conversion of the toxic hydrogen sulfide to elemental sulfur and water is achieved by the modified Claus process whereby the hydrogen sulfide is first oxidized with a stoichiometric amount of air in a reaction furnace at approximately 1200.degree. C. Elemental sulfur, in an amount of 50-60% of the total sulfur content, is formed with the water, sulfur dioxide, carbonyl sulfide and carbon disulfide. The product stream is cooled to about 150.degree. C. so that the elemental sulfur is condensed and can be removed. The remaining gas, bearing a stoichiometric ratio of H.sub.2 S to SO.sub.2 of 2:1, is then fed to a series of adiabatic Claus catalytic converters, where the Claus reaction (1) is practiced. ##STR1##
Usually two or three converters, or reactors, are assembled in series, each equipped with a sulfur condensing unit at its outlet. The temperature within each reactor is regulated by the temperature of the ingoing gases which are heated by on-line burners and by the exothermic nature of the reaction. The temperatures of the 1st, 2nd and 3rd converters are commonly set at approximately 330.degree. C., 250.degree. C. and 180.degree. C., respectively.
The high temperature of the first converter effects the oxidation of the carbonyl sulfide and carbon disulfide to elemental sulfur but, otherwise, the equilibrium conversion through the Claus reaction (1) is low. Therefore, the temperature of the subsequent converters are lowered in order to encourage the equilibrium conversion to elemental sulfur.
An alumina-based catalyst is generally employed in each of the converters. The catalyst originally used was bauxite. However, its effectiveness was often reduced by its poor mechanical strength which resulted in unwanted packing and blocking within the converter. The catalyst which has now found widespread use is activated alumina which is 98% by weight Al.sub.2 O.sub.3. It is available as porous granules, 4-10 mm in diameter, with a surface area of approximately 300 m.sup.2 /gm.
The activity of the catalyst deteriorates through use due to a deposition of carbon and condensed sulfur, and the formation of aluminum sulfate and, in some cases, aluminum nitrates. The catalyst becomes discolored; its surface area drops 90-140 m.sup.2 /gm. Table I shows the chemical analysis of both a fresh and a typical poisoned alumina catalyst.
TABLE I ______________________________________ % Wt. Component Fresh Poisoned ______________________________________ Al.sub.2 O.sub.3 93.60 59.1 Fe.sub.2 O.sub.3 0.02 12.1 Na.sub.2 O 0.30 0.5 SiO.sub.2 0.02 1.5 H.sub.2 O 6.0 14.6 Total sulfur as SO.sub.4.sup.= trace 12.1 ______________________________________
The catalytic rate of conversion of hydrogen sulfide to sulfur is much decreased in the poisoned catalyst. The catalyst must be regenerated or discarded for a fresh batch. The common method of catalyst regeneration is as follows:
The catalyst in the first and second converters is heated to 400.degree.-500.degree. C. and 300.degree.-350.degree. C., respectively, while a dilute H.sub.2 S/SO.sub.2 mixture in the ratio of 2:1 is passed over the catalyst for approximately 24-36 hours in order to remove the condensed sulfur.
Carbon, resulting from the cracking and polymerization of hydrocarbons in the reaction furnace and deposited over the catalyst, is stripped through an oxidative burn-off during which the temperature may be increased to 600.degree. C. Air is fed into the converters via the on-line burners so that the oxygen fraction reacts to form carbon dioxide EQU C+O.sub.2 .fwdarw.CO.sub.2 ( 2)
During this burn-off any residual sulfur may react to form aluminum sulfate which, in turn, must be removed. An H.sub.2 S/SO.sub.2 mixture, approximately 2:1 in composition, is passed over the catalyst for approximately 14 hours to produce alumina and elemental sulfur, the latter being swept out of the converter by the flowing gases. The converter is cooled down to its operating temperature while maintaining a H.sub.2 S/SO.sub.2 ratio of 4:1.
The regeneration process described above is a lengthy one, since the catalyst is not transferred out of the converters. These vessels are heavily insulated and, hence, heating or cooling is very slow.